In general, sintered ore as a main raw material for a blast furnace iron-making method is produced through the process as shown in FIG. 1. The raw material for the sintered ore includes iron ore powder, under-sieve fines of sintered ore, recovery powder generated in the ironworks, a CaO-containing auxiliary material such as limestone, dolomite or the like, a granulation auxiliary agent such as quicklime or the like, coke powder, anthracite and so on, which are cut out from respective hoppers 1 onto a conveyer at a predetermined ratio. The cut-out raw materials are added with a proper amount of water, mixed and granulated in drum mixers 2 and 3 to form quasi-particles having a mean particle size of 3˜6 mm as a sintering raw material. Then, the sintering raw material is charged onto a pallet 8 of a continuous type sintering machine at a thickness of 400˜800 mm from surge hoppers 4 and 5 disposed above the sintering machine through a drum feeder 6 and a cutout chute 7 to form a charged layer 9 also called as a sintering bed. Thereafter, carbonaceous material in a surface part of the charged layer is ignited by an ignition furnace 10 disposed above the charged layer 9, while air above the charged layer is sucked downwardly through wind boxes 11 located just beneath the pallet 8 to thereby combust the carbonaceous material in the charged layer sequentially, and the sintering raw material is melted by combustion heat generated at this time to obtain a sintered cake. The thus obtained sintered cake is then crushed, granulated and agglomerates of about not less than 5 mm in size are collected as a product sintered ore and supplied into the blast furnace.
In the above production process, the carbonaceous material in the charged layer ignited by the ignition furnace 10 is thereafter continuously combusted by air sucked from top down through the charged layer to form a combustion·molten zone having a certain width in a thickness direction (hereinafter referred to as “combustion zone”). The molten portion of the combustion zone obstructs the flow of the sucked air, which is a factor of causing an extension of the sintering time to decrease productivity. Also, the combustion zone is gradually moved from the upper part to the lower part of the charged layer as the pallet 8 moves downstream, and a sintered cake layer finishing the sintering reaction (hereinafter referred to as “sintering layer”) is formed in a portion after passing the combustion zone. Further, as the combustion zone is transferred from the upper part to the lower part, moisture included in the sintering raw material is vaporized by combustion heat of the carbonaceous material and condensed into the sintering raw material in the lower part not yet raising the temperature to form a wet zone. When the water concentration exceeds a certain degree, voids among the particles of the sintering raw material as a path of the gas sucked are filled with water, which is a factor of increasing airflow resistance like the molten zone.
The production volume by the sintering machine (t/hr) is generally determined by productivity (t/hr·m2)×area of the sintering machine (m2). That is, the production volume by the sintering machine is varied depending on width and length of the sintering machine, thickness of a charged layer of the raw material, bulk density of the sintering raw material, sintering (combustion) time, yield and the like. To increase the production volume of the sintered ore, therefore, it is believed that it is effective to shorten the sintering time by improving air permeability of the charged layer (pressure loss) or to increase the yield by increasing the cold strength of the sintered cake before crushing.
FIG. 2 shows distributions of pressure loss and temperature in the charged layer when a combustion zone moving in the charged layer of 600 mm in thickness is located at a position of about 400 mm above the pallet in the charged layer (200 mm below the surface of the charged layer). The pressure loss distribution shows 60% in the wet zone and 40% in the combustion zone.
FIG. 3 shows a transition of temperature and time at a certain point in the charged layer at high and low productivity of the sintered ore, or at fast and slow moving speed of a pallet in the sintering machine, respectively. The time kept at a temperature of not lower than 1200° C. starting the melting of sintering raw material particles is represented by T1 in the low productivity and T2 in the high productivity, respectively. In the high productivity, the moving speed of the pallet is fast so that the high-temperature keeping time T2 becomes short as compared to T1 in the low productivity. However, as the time kept at a high temperature of not lower than 1200° C. is shortened, the sintering becomes insufficient. Hence, the cold strength of the sintered ore is decreased to lower the yield. Consequently, to produce the high-strength sintered ore in a short time with a high yield and a good productivity, it is required to take some measures to prolong the time kept at a high temperature of not lower than 1200° C. to increase the cold strength of the sintered ore.
FIG. 4 is a schematic view illustrating a process wherein the carbonaceous material in the surface part of the charged layer ignited by the ignition furnace is continuously combusted by the sucked air to form the combustion zone, which is moved from the upper part to the lower part of the charged layer sequentially to form the sintered cake. Also, FIG. 5(a) is a schematic view illustrating a temperature distribution when the combustion zone exists in each of an upper part, a middle part and a lower part of the charged layer within a thick frame shown in FIG. 4. The strength of the sintered ore is affected by the product of the temperature of not lower than 1200° C. and the time kept at this temperature, and as the value becomes larger, the strength of the sintered ore becomes higher. Accordingly, the middle and lower parts in the charged layer are pre-heated by combustion heat of the carbonaceous material in the upper part of the charged layer carried with the sucked air and thus kept at a high temperature for a long time, whereas the upper part of the charged layer is lacking in the combustion heat due to no preheating. Hence, combustion melting reaction required for sintering (sintering reaction) is liable to be insufficient. As a result, the yield of the sintered ore in the widthwise section of the charged layer becomes smaller at the upper part of the charged layer as shown in FIG. 5(b). Moreover, both widthwise end portions of the pallet are supercooled due to heat dissipation from the side walls of the pallet or a large amount of air passed so that the high-temperature keeping time required for sintering cannot be secured sufficiently and the yield is also lowered.
As to these problems, it has hitherto been performed to increase the amount of the carbonaceous material (powdery coke) added in the sintering raw material. However, it is possible to raise the temperature in the sintered layer and prolong the time kept at not lower than 1200° C. by increasing the addition amount of coke as shown in FIG. 6, while at the same time, the maximum achieving temperature in the sintering exceeds 1400° C. and the decrease of the reducibility and cold strength of the sintered ore is caused by the reason as described below.
In Table 1 of ‘Mineral engineering’, edited by Hideki IMAI, Sukune TAKENOUCHI, Yoshinori FUJIKI, (1976), p. 175, Asakura Publishing Co., Ltd. are shown tensile strength (cold strength) and reducibility of various minerals generated in the sintered ore during the sintering. In the sintering process, a melt starts to be generated at 1200° C. to produce calcium ferrite having the highest strength and a relatively high reducibility among constitutional minerals of the sintered ore as shown in FIG. 7. This is the reason why the sintering temperature is required to be not lower than 1200° C. However, when the temperature is further raised and exceeds 1400° C., precisely 1380° C., calcium ferrite starts to be decomposed into an amorphous silicate (calcium silicate) having the lowest cold strength and reducibility and a secondary hematite of a skeleton-crystal form easily causing reduction degradation. Also, the secondary hematite constituting a start point of the reduction degradation of the sintered ore raises the temperature up to a zone of Mag. ss+Liq. and is precipitated in the cooling as shown in a phase diagram of FIG. 8 from the results of the mineral synthesis test so that production of the sintered ore through a path (2) instead of a path (1) shown in the phase diagram is considered to be important to suppress the reduction degradation.
TABLE 1Type of mineralTensile strength (MPa) Reducibility (%)Hematite4950Magnetite5822Calcium ferrite10235Calcium silicate193
That is, ‘Mineral engineering’, edited by Hideki IMAI, Sukune TAKENOUCHI, Yoshinori FUJIKI, (1976), p. 175, Asakura Publishing Co., Ltd. discloses that in controlling the maximum achieving temperature, the high-temperature keeping time and the like during combustion is a very important control item to ensure the quality of the sintered ore and the quality of the sintered ore is substantially determined depending on these controls. Therefore, to obtain a sintered ore having a high strength and excellent reduction degradation index (RDI) and reducibility, it is important that calcium ferrite produced at a temperature of not lower than 1200° C. is not decomposed into calcium silicate and secondary hematite. To this end, it is necessary that the maximum achieving temperature in the charged layer during sintering does not exceed 1400° C., preferably 1380° C., while the temperature in the charged layer is kept at not lower than 1200° C. (solidus temperature of calcium ferrite) for a long time. The time kept in the temperature range of not lower than 1200° C., but not higher than 1400° C. is hereinafter called as “high-temperature keeping time”.
Moreover, there are proposed some techniques for the purpose of keeping the upper part of the charged layer at a high temperature for a long time. For example, JP-A-S48-018102 proposes a technique of injecting gaseous fuel onto the charged layer after the ignition of the charged layer, JP-B-S46-027126 proposes a technique of adding a flammable gas to air sucked into the charged layer after ignition of the charged layer, JP-A-S55-018585 proposes a technique wherein a hood is disposed above the charged layer and a mixed gas of air and coke oven gas is jetted from the hood at a position just behind the ignition furnace to make the temperature in the charged layer of the sintering raw material higher, and JP-A-H05-311257 proposes a technique of simultaneously blowing a low-melting point flux and carbonaceous material or flammable gas at a position just behind the ignition furnace.
In those techniques, however, since gaseous fuel with a high concentration is used and the amount of the carbonaceous material is not decreased in blowing the gaseous fuel, the maximum achievable temperature of the charged layer in the sintering becomes high exceeding 1400° C. as an upper limit temperature under operation control so that calcium ferrite produced in the sintering process is decomposed to form a sintered ore having low reducibility and cold strength. Hence, the effect of improving the yield is not obtained or the air permeability is deteriorated due to the temperature rising and thermal expansion by the combustion of the gaseous fuel to decrease the productivity, or further there is a risk of causing fire accident in the upper space of the sintering bed (charged layer) with the use of the gaseous fuel. As a result, any of these techniques are not brought into practical use.
When the techniques disclosed in WO 2007/052776, JP-A-2010-047801, JP-A-2008-291354 and JP-A-2010-106342 are applied to the method of producing the sintered ore with the downdraft type sintering machine to decrease the amount of the carbonaceous material added to the sintering raw material and further the gaseous fuel diluted to not higher than the lower limit concentration of combustion is introduced into the charged layer to combust the gaseous fuel in the charged layer as shown in FIG. 9, the gaseous fuel is combusted in the charged layer (in the sintering layer) after combustion of the carbonaceous material so that the width of the combustion·molten zone can be enlarged into the thickness direction without exceeding the maximum achieving temperature over 1400° C. and hence the high-temperature keeping time can be prolonged.
To produce the high-quality sintered ore having a high strength and an excellent reducibility in a high yield, it is required to ensure the time kept at a high temperature range of not lower than 1200° C. but not higher than 1400° C. (high-temperature keeping time) at least for not less than the predetermined time, whereas even if the keeping time is prolonged excessively from the predetermined time, the effect is saturated. To this end, the high-temperature keeping time is desirable to be not less than the predetermined value and uniform over the full area of the charged layer in the thickness direction as shown by a dashed line in FIG. 10. However, the techniques of WO 2007/052776, JP-A-2010-047801 and JP-A-2008-291354 have an effect of uniformizing the high-temperature keeping time in an area getting inside from the surface portion of the charged layer of the sintering raw material to a certain level as shown in FIG. 10, while it is difficult to ensure the high-temperature keeping time of not less than the predetermined value in an area ranging from the surface of the raw material charged layer to about 30% of the layer thickness because the carbonaceous material is decreased in the operation by supplying the gaseous fuel and further the area is cooled by air introduced into the charged layer. Therefore, the yield in the surface portion of the raw material charged layer is somewhat improved by the supply of the gaseous fuel, but the effect is limited. The technique of JP-A-2010-106342 proposes that the concentration of the diluted gaseous fuel to be supplied is made higher in an upstream side of the supplied area than that in the downstream side in the operation by supplying the gaseous fuel. However, the area ranging from the surface of the raw material charged layer to about 30% of the layer thickness is cooled by air introduced into the charged layer after the ignition, so that the high-temperature keeping time cannot be ensured sufficiently, and consequently the effect by supplying the gaseous fuel into the surface portion of the raw material charged layer is limited as in WO 2007/052776, JP-A-2010-047801 and JP-A-2008-291354.
We developed a technique of intensively supplying the gaseous fuel into such an area of the raw material charged layer that the time kept at a high temperature range of not lower than 1200° C. (high-temperature keeping time) is less than 150 seconds in case of sintering by combustion heat of only the carbonaceous material, the result of which is filed as Japanese Patent Application No. 2011-054513. In that technique, however, although the length of the gaseous fuel supplied (supplying position) is varied, when the concentration of the gaseous fuel supplied is constant or when the concentration of the gaseous fuel is made higher in the upstream side of the supplied area than that in the downstream side thereof as described in JP-A-2010-106342, it is actual that the maximum achieving temperature in the sintering is still lower than 1200° C. in the outermost surface portion within 100 mm from the surface of the raw material charged layer, or even if it reaches the above value, the high-temperture keeping time is difficult to be ensured for a long time.
It could therefore be helpful to provide a method of producing a sintered ore wherein the time kept at a high-temperature range is stably ensured even in the outermost surface portion of the sintering raw material charged layer and hence a high-quality sintered ore having a high strength and an excellent reducibility can be produced in a high yield.